This invention relates to refrigerant compressors driven by an electrical motor, and more particularly relates to means for controlling a refrigerant compressor utilizing a two-speed, single phase electrical motor.
Refrigerant compressors utilizing a two-speed compressor motor have been devised in the past. One such arrangement is shown in U.S. Pat. No. 3,584,980 (Cawley et al -- June 15, 1971). Another is described in the applicants' U.S. application Ser. No. 436,179 entitled "Control Apparatus for Two-Speed Compressor", filed Jan. 24, 1974. Although a two speed compressor results in improved performance, the applicants have discovered that such an arrangement requires precise control in order to prevent damage to the compressor motor during two speed operation.
Compressor motors adapted to operate from a single phase source of electrical power generally comprise a number of windings. The applicants have found that each of these windings must be protected from thermal damage in order to ensure long motor life. Since thermal damage can occur rapidly under certain adverse operating conditions, the windings must be protected by sensitive temperature detectors capable of rapid response.
Although one temperature sensor placed adjacent to one of the windings will tend to provide protection against thermal destruction due to a locked rotor or running overload, a single sensor will not give adequate protection under all operating conditions. The applicants have discovered that protection is enhanced by providing a dual sensor for the windings in the motor. These sensors provide an indication of the temperature adjacent each winding. A current transformer is used in combination with the dual temperature sensors to protect the windings. If one of the sensors indicates an abnormally high temperature in an associated winding, or if the current sensing device or current transformer senses an abnormally high input current, control means operate a switch in a motor pilot circuit in order to remove all electrical power from the motor. In this manner, the apparatus provides complete overload protection.
Accordingly, it is an object of the present invention to provide a hermetic refrigerant compressor in which a motor adapted to operate on single phase electrical power has temperature sensors located adjacent the motor windings, so that each winding in the motor is adequately protected against thermal damage.
It is still another object of the invention to provide control apparatus of the foregoing type in combination with current sensing means for sensing abnormally high current conduction in any motor windings.
It is still another object of the invention to provide control apparatus of the foregoing type; in which a control device interrupts power to the motor if any one of the sensors detects an abnormally high temperature or high current in any of the motor windings.
The applicants have also discovered that the control of a two-speed compressor motor requires careful design of the contactors used to switch from a high speed mode of operation to a low speed mode of operation and vice versa. In a compressor motor adapted to be operated from single phase electrical power, multiple windings are frequently provided and must be interconnected in different ways in order to achieve different speeds of operation. The applicants have invented a unique system of electrically and mechanically interlocking the contactors required for two-speed operation in order to assure a failsafe system of operation.
It is still another object of the present invention to provide motor control apparatus of the foregoing type in which both mechanical and electrical interlocks are operated by and simultaneously with the contactors controlling the two-speed operation of the motor.